Role of protic ionic liquid concentration in proton conducting polymer electrolytes for improved electrical and thermal properties

Protic ionic liquids (PILs) in the acidic medium are known to show higher ionic conductivity than neat PIL or PIL in alkaline media. Hence, polymer electrolyte membranes (PEM) containing both PIL and acids are considered ideal for non-humidified intermediate temperature PEM fuel cells. Herein, we report non-aqueous proton conducting PEM made up of diethylmethylammonium trifluoromethanesulfonate; [dema][TfO] and neat phosphoric acid (H₃PO₄) with poly (vinylidene fluoride-co-hexafluoropropylene); PVDF-HFP as the host matrix. The presence of PIL significantly modified the structure and microstructure of the electrolyte films with the emergence of micropores in the PIL containing membranes. SEM images suggest leaching of PIL and phosphoric acid above 80 wt% of PIL in the electrolyte membranes. Thermogravimetric studies show that the dehydration in the PEM films due to phosphoric acid condensation at 100 °C-200 °C region is arrested by the presence of PIL. The maximum ionic conductivity at room temperature is ∼6.3 × 10^-4 S cm^-1 at 40 wt% of [dema][TfO] addition, which is two orders higher than that of the primary electrolyte (PE) containing only phosphoric acid in PVDF-HFP. This high conductivity in PEM films can be correlated to the increase in polar β and γ phases as well as a drop in the total crystallinity fraction in the film. The study using dielectric spectroscopy reveals a strong coupling of ionic conductivity with the structural or segmental relaxation of the PVDF-HFP due to the presence of [dema][TfO] in the PEMs.